In recent years, there have been increasing demands for higher strength in the steel sheet which is used for automobiles etc. In particular, for the purpose of improving collision safety etc., high strength steel sheet with a tensile maximum stress of 900 MPa or more is also being used. Such high strength steel sheet is inexpensively formed in large volumes by press working it in the same way as soft steel sheet and is being used as structural members.
However, in recent years, along with the rapid increase in strength of high strength steel sheet, in particular in high strength steel sheet with a tensile maximum stress of 900 MPa or more, the problem has arisen of the shapeability becoming insufficient and of working accompanied with local deformation such as stretch-formability becoming difficult. Further, when a high speed tension force acts on a steel material, there was the problem that the fracture mode would easily change from ductile fracture to brittle fracture.
In the past, as one example of the art for strengthening a steel material, a high strength steel material which was hardened by causing the fine precipitation of Cu was known. PLT 1 discloses a Cu precipitation hardening type high strength steel material which contains C, Si, P, S, Al, N, and Cu in predetermined ranges, contains one or both of Mn: 0.1 to 3.0% and Cr: 0.1 to 3.0%, has an (Mn+Cr)/Cu of 0.2 or more, and has a balance of iron and unavoidable impurities, has an average ferrite crystal grain size of 3 μm or more, and has a ferrite area ratio of 60% or more.
Further, as one example of high strength steel sheet which achieves both shapeability and hole expandability, PLT 2 discloses high strength steel sheet which is excellent in shapeability and hole expandability which contains C, Si, Cu, and Mn in predetermined mass %, further suitably contains at least one of Al, Ni, Mo, Cr, V, B, Ti, Nb, Ca, and Mg, and has a hardness of the ferrite phase of Hv 150 to 240, has a volume ratio of residual austenite in the steel structure of 2 to 20%, and exhibits a tensile strength of 600 to 800 MPa.
PLT 3 discloses, as one example of high strength cold rolled steel sheet for working use which is excellent in fatigue characteristics, high strength cold rolled steel sheet for working use which is excellent in fatigue characteristics which is comprised of steel sheet containing C: 0.05 to 0.30%, Cu: 0.2 to 2.0%, and B: 2 to 20 ppm and which has a microstructure comprised of a volume ratio of 5% or more and 25% or less of residual austenite and ferrite and bainite and which has Cu present in the ferrite phase in the state of particles which are comprised of Cu alone of a size of 2 nm or less in a solid solution state and/or precipitated state.
PLT 4 discloses, as one example of composite structure high strength cold rolled steel sheet which is excellent in fatigue characteristics, composite structure high strength cold rolled steel sheet which is comprised of ferrite-martensite composite structure steel sheet which contains C: 0.03 to 0.20%, Cu: 0.2 to 2.0%, and B: 2 to 20 ppm and which has Cu present in the ferrite phase in the state of particles which are comprised of Cu alone of a size of 2 nm or less in a solid solution state and/or precipitated state.
PLT 5 discloses, as one example of super high strength steel sheet which is excellent in delayed fracture resistance, super high strength steel sheet which contains, by wt %, C: 0.08 to 0.30, Si: less than 1.0, Mn: 1.5 to 3.0, S: 0.010 or less, P: 0.03 to 0.15, Cu: 0.10 to 1.00, and Ni: 0.10 to 4.00, has a balance of iron and unavoidable impurities, contains one or more structures of martensite, tempered martensite, or bainite structures in a volume ratio of 40% or more, and has a strength of 1180 MPa or more.
PLT 6 discloses, as one example of high strength steel sheet which is excellent in press formability and corrosion resistance, high strength steel sheet which is excellent in press formability and corrosion resistance which satisfies the requirements of C: 0.08 to 0.20%, Si: 0.8 to 2.0%, Mn: 0.7 to 2.5%, P: 0.02 to 0.15%, S: 0.010% or less, Al: 0.01 to 0.10%, Cu: 0.05 to 1.0%, and Ni: 1.0% or less, has a balance of iron and unavoidable impurities, and satisfies the relationship of the following formula “0.4≦(10P+Si)/(10C+Mn+Cu+0.5Ni)≦1.6” (wherein, the notations of elements indicate the respective contents (%)), which steel sheet has residual austenite of 3 to 10% and a tensile strength of 610 to 760 MPa.
PLT 7 discloses, as one example of high strength thin gauge steel sheet, high strength thin gauge steel sheet which has a composition of ingredients which contains C: 0.05 to 0.3%, Si: 2% or less, Mn: 0.05 to 4.0%, P: 0.1% or less, S: 0.1% or less, Cu: 0.1 to 2%, and Si (%)/5 or more, Al: 0.1 to 2%, N: 0.01% or less, Ni: Cu (%)/3 or more (when Cu is 0.5% or less, not necessarily included) and satisfies “Si (%)+Al (%)≧0.5”, “Mn (%)+Ni (%)≧0.5”, has a structure which contains a volume ratio of 5% or more of residual austenite, and exhibits a tensile strength of 650 to 800 MPa.